Liquid metal monitor method



p 1967 K. WILKINSON LIQUID METAL MONITOR METHOD 5 Sheets-Sheet l Filed Dec.

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LIQUID METAL MONITOR METHOD Filed Dec. 1964 s Sheets-Sheet 'F/G.4. 42 Z 78 2 l F/G.6

United States Patent 3,340,725 LIQUID METAL MONITOR METHOD Keith Wilkinson, Thurso, Caithness, Scotland, assignor to United Kingdom Atomic Energy Authority, London, England Filed Dec. 3, 1964, Ser. No. 415,583 Claims priority, application Great Britain, Dec. 10, 1963, 48,832/ 63 2 Claims. (Cl. 7361) This invention relates to liquid metal and more particularly to a method for operating such monitors, monitors such as are employed to determine the purity of a liquid metal. Liquid metal impurity measurement is particularly important where the liquid metal is to be used as a coolant or heat transfer agent in a heat ex change circuit since impurities dissolved in the liquid metal can accelerate the rate of corrosion of materials in contact with the liquid metal and can also cause difiiculty by precipitation of the impurities at cool sections of the circuit. Liquid sodium is a conventional liquid metal coolant which is liable to contamination by sodium oxide soluble impurity.

In one range of liquid metal monitors, known as plugging meters, reliance is placed upon precipitation of an impurity at a measured temperature so as to obstruct or plug a restriction in a circuit through which the liquid metal is passed. By reference to the temperature-solubility relationship for that impurity, the temperature of plugging which is measured can be related to an impurity concentration in the liquid metal. These instru ments are conventionally operated by alternately plugging and unplugging the restriction. It has been proposed, however, that these instruments should be operated on a continuous basis by establishing a partial plugging of the restriction and preserving a steady but reduced flowrate of liquid metal through the restriction. Under such conditions the liquid metal at the restriction is saturated with the soluble impurity so that substantially no precipitate is added to or subtracted from the deposit at the restriction. Operation of a plugging meter in this partially plugged condition is dependent upon close control and measurement of the variables in the instrument, namely liquid metal temperature and flow rate, and development of the instrument is directed to this end.

The present invention provides, in one of its aspects, a method of continuously determining the purity of a liquid metal including the steps of passing a continuous stream of liquid metal through a duct having a restrictor therein adapted to be partially plugged by the deposit of precipitated impurity, continuously subjecting the liquid metal upstream of the restrictor to forced cooling while simultaneously maintaining but selectively varying an input of heat to the liquid metal to vary the cooling effect of the forced cooling by controlled variation of the electrical heating, whereby a deposit of impurity precipitated by cooling of the liquid metal is maintained substantially without addition or subtraction and where the temperature of the liquid metal under these conditions is related to the impurity concentration of the liquid metal.

Embodiments of liquid metal monitor in accordance with the invention will now be described by way of "ice example with reference to the accompanying diagrammatic drawings in which:

FIGURE 1 is a sectional elevation of one form of the monitor,

FIGURE 2 is an enlarged section on line IIII of FIGURE 1,

FIGURE 3 is a sectional elevation of an alternative embodiment of the monitor,

FIGURE 4 is an enlarged fragmentary plan view of an alternative to part of FIGURE 1, and

FIGURES 5 and 6 are sectional views of alternative arrangements of parts of FIGURE 1.

The liquid metal monitor shown in FIGS. 1 and 2 embodies a long, upright duct 11 through which liquid metal is caused to flow in a downward direction. In service the duct is typically connected into a liquid sodium flow circuit, such as the coolant circuit of a heat exchange system, and a continuous stream of the liquid sodium coolant is bled through the duct. Towards the lower end of the duct, there is a flow restrictor 12 which is designed to be of small size so as to have a low thermal capacity in order to improve the sensitivity of the monitor. The restrictor 12 is combined with a thermocouple 13 by which the temperature of liquid metal at the restriction is indicated. The restrictor 12 comprises a wall 14 formed as an internal annular projection of the duct 11 and in which the hot junction of the thermocouple 13 is embedded. A central aperture 16 in the wall 14 is formed with four grooves 17 (FIGURE 2) in its periphery. A needle 18 is shown located with its .point entered into the aperture 16 to block the aperture whilst leaving the grooves 17 open to' liquid metal fiowing through the duct. The aperture 16 typically has a diameter 2.5 times greater than that of the grooves 17. With the needle 18 in this normal position, the restrictor 12 closes the duct 11 to liquid flow apart from four orifices provided by the four grooves 17. Flowrate measuring means in the form of a liquid metal flowmeter 19 of the electromagnetic type is mounted upstream of the resistor to indicate the fiowrate of liquid metal through the duct 11 and restrictor 12.

The needle 18 is axially movable with respect to the restrictor 12 so that the restrictor orifices can be enlarged to clean them if they become blocked by deposits from the liquid metal. To this end the needle 18 is secured at its upper end to an armature 21 which is located within a fixed solenoid 22 mounted externally on the duct 11. The needle can be raised away from the restrictor, against the action of gravity, by energisation of the solenoid.

A liquid metal temperature conditioner is located upstream of the restriction aflorded by the restricter 12. In order to permit a close and rapid control of the liquid metal temperature and its rate of change, the conditioner is adapted to regulate the liquid metal temperature by a combination of electrical heating and forced cooling. Forced cooling of the liquid sodium is achieved by a cooling airstream which is circulated upwardly by a fan (not shown) through a coaxial shroud 23 housing the liquid metal duct 11. Within the shroud and between its inlet and outlet sections, the liquid metal duct 11 carries fins 24 which enhance the cooling efiect of the airstream on the liquid metal. In the monitor now described the electrical heating is by means in the form of an electrical heating coil 25 wound around the duct on the inlet side of the restrictor and with its turns in close contact with the duct 11 between the heat exchange fins 24. Alternatively, electrical heating of liquid metal in the duct 11 may be by means of the passage of a low voltage electrical current through the liquid metal between electrodes 26 embedded in the walls of the duct 11. The response of the liquid metal temperature to variations in electrical heating by a coil or by passage of current through the metal is found to be rapid and predictable with accuracy. Although temperature regulation by adjustment of the velocity of the cooling airstream is possible, for example by adjustment of the fan velocity or of louvres in the airstrearn, this regulation tends to be imprecise owing to irregular characteristics in the performance of the fan or the louvres. In the present instrument the rate of forced cooling is held substantially constant and a variable rate of electrical heating is applied to the liquid metal to offset the uniform cooling rate to a variable degree. It is possible by means of the combination of electrical heating and forced cooling to establish selected rates of liquid metal temperature increase or decrease over a wide range.

The monitor is provided with a control unit in which conventional electronic techniques are employed to achieve the desired programme for operation of the monitor. Inputs to the control unit are signals from the flowmeter 19 and the thermocouple 13 whilst outputs from the control unit control the electrical heating coil 25 and the solenoid 22. The control unit is programmed to cause a steady preselected rate of decrease in temperature of the liquid metal. When the flow meter signals a preselected reduction, say 25%, in the liquid metal flowrate, the control unit regulates the energisation of the heating coil to maintain the flowrate steady at this reduced rate. Under these conditions (as explained above) the monitor is operating in a partially plugged state with saturation conditions at the restrictor. The temperature indicated by the thermocouple corresponds to the saturation temperature of the solution of impurity in the liquid metal and can readily be related to the impurity concentration in the liquid metal.

The control unit is programmed to maintain saturation conditions at the restrictor notwithstanding variations in the impurity concentration in the liquid metal passed through the monitor; these variations are indicated as saturation temperature variations by the thermocouple. In the event that the rest 'rictor becomes wholly blocked by impurity deposits, the control unit is programmed to clear the blockage by raising the liquid metal temperature and by energising the solenoid to raise the needle to unblock the aperture 16. The aperture 16 is typically 0.125 in diameter whilst the grooves 17 have a diameter of 0.052 on a 0.156" P.C.D.

FIGURE 3 (in which like parts to those of FIGS. 1 and 2 are designated by similar reference numerals) shows an alternative embodiment of monitor which has two spaced coaxial ducts 30, 31, the inner duct 30 being open at its lower end and the outer duct 31 being closed at its lower end by an end plate 32. The inner duct 30 has the internal restrictor 12 formed by the wall 14, and flow of liquid sodium is downwardly in the annulus between the ducts 30, 31 and then upwardly through the restrictor 12 and duct 30. The plain outer wall of the duct 30 supports the helically wound heating coil 25 whilst the duct 31 is provided with the external heat exchange fins 24. The shroud 23 round the ducts 30, 31 has a fan 33 for circulation of air upwardly through the annulus between the duct 31 and the shroud 23. The duct 31 has a drain valve V. The fan constitutes a form of forced cooling means.

Operation is similar to that of the FIGS. 1 and 2 embodiment, the arrangement of FIG. 3 providing that the heating coil 25 is immersed in liquid sodium on the inlet side of the restrictor 12, i.e. in the liquid sodium flowing downwardly in the annulus between the ducts 30-, 31, thus improving the response rate, that is to say the rate at which the temperature of the liquid sodium follows variations in the electrical heating current. This compares with the arrangement of FIG. 1 in which heating of the liquid metal on the inlet side of the restrictor is by heat transfer from the coil 25 through the wall of the duct 11.

It will be understood that liquid metal monitors which are to be operated with saturation conditions at a partially plugged restriction have a sensitivity which is dependent upon the accuracy of measurement of the liquid metal temperature and flowrate. As described above accuracy of measurement of the liquid metal temperature is achieved by combining the thermocouple 13 with the restrictor 12. This concept may be further developed by constructing the restrictor itself of the materials of the thermocouple. In this case the restrictor wall is divided, for instance, at its central radial plane, into adjoining halves 40, 41 (FIG. 4, which is an enlarged fragmentary plan view of the restrictor) of dissimilar thermocouple materials. A thermocouple junction 42 is formed at the joint between the two halves and this junction, being inte-' gral with the orifices in the restrictor, is extremely sensitive to the temperature of liquid metal at the restrictor. The halves 40, 41 may for example be of platinum and platinum/rhodium alloy respectively, and be joined as by electron beam welding. With a similar aim, namely to concentrate measurement of the liquid metal variables at the restrictor 12, the flowmeter 19 upstream of the restrictor may be replaced by two pressure transducers 43 (FIG. 5), one on each side of and adjacent to the restrictor, and thus to measure a pressure drop across the restrictor as an indication of liquid metal flowrate through the restrictor. The transducers 43 may for example be of the electrical capacitance type, that is to say having an inner cylindrical electrode and an outer tubular electrode on which the pressure acts externally to deform it elastically, and a composite solid and gaseous annular dielectric layer between the electrodes. The solid dielectric is preferably of refractory material such as alumini-um oxide deposited on the inner electrode and the elastic deformation of the outer surrounding tubular electrode varies the thickness of the gaseous dielectric and hence the capacitance of the transducer. Alternatively the liquid metal flowrate may be measured by a flowmeter at the restrictor. Thus FIG. 6 shows two diametrically opposed magnets 44 of an electromagnetic type of flowmeter, poles 45 of the magnets being disposed in close proximity to the restrictor 12. The magnets 44 are located in a keeper ring 46 positioned from a normally sealed access port 47 through which electrical connections pass. The liquid metal flow duct is shown in FIG. 6 for containment purposes to be of double walled construction having an inner duct 11 and an outer duct 11 so that any leakage from the duct 11 is into the duct 11 Also shown is an outlet pipe 48'.

I claim:

1. In a method of operating a liquid metal monitor continuously to determine the purity of a liquid metal, the monitor comprising a duct for flow of the liquid metal, a restrictor in the duct adapted to be partially plugged by the deposit of precipitated impurity, and a temperature conditioner associated with a section of the duct immediately upstream of the restrictor, the conditioner comprising forced cooling means and variable electrical heating means, the steps comprising passing a continuous stream of liquid metal through said duct and said restrictor, and continuously operating said forced cooling means while simultaneously maintaining but selectively varying the output of said electrical heating means so as to controllably vary the cooling effect of the cooling means by variation of the heating means, whereby a deposit of impurity precipitated by cooling of the liquid metal is maintained substantially without addition or subtraction and where the temperature of the liquid metal under these conditions is related to the impurity concentration of the liquid metal.

2. In a method of continuously determining the purity of a liquid metal, the steps comprising passing a continuous stream of liquid metal through a duct having a restrictor therein adapted to be partially plugged by the deposit of precipitated impurity, continuously subjecting the liquid metal upstream of the restrictor to forced cooling while simultaneously maintaining but selectively varying an input of heat into the liquid metal to vary the cooling etfect of the forced cooling by controlled variation of the electrical heating, whereby a deposit of impurity precipitated by cooling of the liquid metal is maintained substantially without addition or subtraction and conditions is related to the impurity concentration of the liquid metal.

References Cited UNITED STATES PATENTS 2,922,305 l/1960 Wehrman 73-359 X 2,997,874 8/1961 Billuris et a1 7361 3,200,637 8/ 1965 Ballou et a1. 7361 3,222,916 12/ 1965 Davis.

FOREIGN PATENTS 230,413 9/ 1960 Australia.

where the temperature of the liquid metal under these 15 DAVID SCHONBERG, Primary Examiner. 

1. IN A METHOD OF OPERATING A LIQUID METAL MONITOR CONTINUOUSLY TO DETERMINE THE PURITY OF A LIQUID METAL, THE MONITOR COMPRISING A DUCT FOR FLOW OF THE LIQUID METAL, A RESTRICTOR IN THE DUCT ADAPTED TO BE PARTIALLY PLUGGED BY THE DEPOSIT OF PRECIPITATED IMPURITY, AND A TEMPERATURE CONDITIONER ASSOCIATED WITH A SECTION OF THE DUCT IMMEDIATELY UPSTREAM OF THE RESTRICTOR, THE CONDITIONER COMPRISING FORCED COOLING MEANS AND VARIABLE ELECTRICAL HEATING MEANS, THE STEPS COMPRISING PASSING A CONTINUOUS STREAM OF LIQUID METAL THROUGH SAID DUCT AND SAID RESTRICTOR, AND CONTINUOUSLY OPERATING SAID FORCED COOLING MEANS WHILE SIMULTANEOUSLY MAINTAINING BUT SELECTIVELY VARYING THE OUTPUT OF SAID ELECTRICAL HEATING MEANS SO AS TO CONTROLLABLY VARY THE COOLING EFFECT OF THE COOLING MEANS BY VARIATION OF THE HEATING MEANS, WHEREBY A DEPOSIT OF IMPURITY PRECIPITATED BY COOLING OF THE LIQUID METAL IS MAINTAINED SUBSTANTIALLY WITHOUT ADDITION OR SUBSTRACTION AND WHERE THE TEMPERATURE OF THE LIQUID METAL UNDER THESE CONDITIONS IS RELATED TO THE IMPURITY CONCENTRATION OF THE LIQUID METAL. 